What is it about the Greek that prohibits this? I ask because brother of the Lord makes perfect sense in English to be taken either metaphorically or literally, as a title or otherwise, there's not a problem, but the Greek appears to prohibit the use of metaphors, at least according to you.
1)First and foremost, it isn't just "brother of the lord." It is James, the brother of the lord. This is important.
And to answer your question, the reason the greek makes it impossible to be a metaphor is because this is a formula to identify a person by way of kin. People didn't have last names. Additionally, too many people had the same name. So they were identified by fathers, brothers, husbands, place of origin, or nick names. The way to identify by kin is X the Y of Z. This is never used as a metaphor, which would defeat the purpose.
2) We see Paul use brother as a metaphor all the time. We see this elsewhere in christian and in greek literature. Using the vocative of
adelphos or simply calling people
adelphos is used metaphorically. Additionally, Paul frequently used the expression
adelphoi en to kyrio/brothers in the lord as a metaphor. We never see "brother of X" let alone "Y the brother of X" as a metaphor.
Would this present serious problems translating this into Greek for the translator and the Greek reader?
Yes, in classical greek. Because this would be identifying him by his kin, and the metaphor would be lost.